1. Field of the Invention
The present invention is directed to a process and apparatus for monitoring dewatering in a wet end section of a paper machine, in particular in the press section.
2. Discussion of Background Information
In the wet end section of a paper machine, e.g., in the press section, the wet fibre web is pressed between cylindrical rolls. Press fabrics are passed through press nips formed by these cylindrical rolls with the web to cushion the web and to absorb water squeezed from the web. A series of press nips acts upon the web before it leaves the press section to pass to the dryer section of the paper machine.
The water removed from the paper web is partly absorbed by the press fabrics, which are generally arranged on each face of the paper web to sandwich the web, and the remainder is expelled mechanically from each nip of the press section to be caught in collection troughs, or through vacuum backed ventilated rolls. After leaving the press section the press fabric is treated with additional water by showers to clean, condition and lubricate the fabric. The press fabric is then passed over a vacuum area formed by one or more suction devices, e.g., Uhle boxes, which causes water to migrate to the fabric surface for mechanical removal.
The press section is intended to remove the maximum amount of water without compromising the quality of the paper web produced. The amount of water removed is dependent on nip pressure, water absorption capacity of the press fabrics, and the ability of the press rolls to carry water away from the press nip to collection troughs.
The water absorption capacity of the press fabric depends upon the water volume received from the cleaning sprays, the water removal capacity of the vacuum dewatering devices, the cleanliness of the press fabric and the design characteristics of the press fabric.
At present it is not possible to know the dryness of the paper web as it leaves each press nip or as it finally enters the dryer section. To measure the water content directly requires that the sheet be broken, and the machine be out of use for a costly period of downtime. Indirect measurement of the water content of the web requires that a material balance be determined by measuring water flow from the collection troughs, vacuum dewatering devices, and the water content remaining in the press fabric. The latter can be measured using a microwave based moisture meter which is manually pressed into the fabric and moved across the width of the machine. In addition the water content entering press section must be measured or assumed.
Because the carrying out of these material balance procedures require expertise and time, they are not normally carried out except under special circumstances. Consequently, during the normal running of the paper machine there are many unknowns which prevent optimization of the water removal process.
The unknowns include, e.g., the effect of press loading on distribution of water flow into the fabric and into the collection trough and overall removal; the effect of the fabric shower flow on the distribution of water flow and overall water removed; the effect of upstream conditions on water removed in the press section; the effect of the paper basis weight on water removal in the press section; the effect of fabric cleanliness on water flow and overall removal; the effect of vacuum levels, dwell time and airflow on water flow and overall removal; the effect of needle jet shower pressures on water flow and overall removal; the effect of roll cover hardness on water flow and overall removal; and the effect of press fabric design on water flow and overall removal.
U.S. Pat. No. 3,655,980 proposes measuring the drainage rate from slurry along the length of a forming wire, using a radiation source and radiation detectors. In U.S. Pat. No. 3,724,957, the concentration of an optically active substance is measured using photoelectric detectors to determine the concentration of pulp and clay in a papermaking slurry, while U.S. Pat. No. 3,860,168 uses a nucleonic detector to monitor the weight of paper sheet. Moisture sensors are used in, e.g., U.S. Pat. Nos. 5,093,795 and 5,262,955, which measure the moisture content profile and adjust the moisture content by adding water or steam to the web, and in, e.g., U.S. Pat. No. 5,286,348, which uses an infra red sensor on the web emerging from the last dryer in a papermaking machine.
U.S. Pat. No. 4,752,356 discloses sampling the slurry at the wet end of a paper machine to determine the total organic carbon present as a measure of the requirement for cationic additive materials to neutralize anionic contaminants in the papermaking process, and U.S. Pat. No. 5,330,621 discloses continuously analyzing cellulose pulp slurry to determine elemental constituents, by gamma neutron activation analysis or carbon content analysis. The measurements are not made on the paper machine during a papermaking process.
However, none of these measurement methods meet the requirements for continuous material balance monitoring during paper machine operation.
International Publication No. WO 00/08462, the disclosure of which is expressly incorporated by reference herein in its entirety, discloses a process and apparatus for monitoring water balance in a paper machine in which the electrical conductivity of the felts entering the press section are compared to the electrical conductivity of the paper web emerging from the press section. However, in this system, wet web (paper volume) flow to the press section is calculated while the solids content of wet web and its conductivity in front of the press is unknown. Further, as the felts are not included in the solids balance, they only apply to the stationary condition.